The short answer: Hill’s Prescription Diet z/d (B/76) beats Hill’s Science Diet (C/61) by 15 points. z/d is a hydrolyzed protein diet for dogs with severe food allergies — the proteins are broken down so small that the immune system can’t react to them. Despite using corn starch as the first ingredient, z/d avoids the cheap plant protein fillers that drag Science Diet down. These two foods serve completely different purposes and shouldn’t be compared as direct alternatives.

The scores

Hill’s Prescription Diet z/d: B (76/100)

Hill’s Science Diet: C (61/100)

A 15-point gap across the grade boundary. z/d earns its B through a specialized formulation that avoids the filler-heavy ingredient patterns of typical Hill’s products.

How the ingredients compare

z/d: Corn Starch, Hydrolyzed Chicken Liver, Hydrolyzed Chicken, Ground Pecan Shells, Powdered Cellulose

Science Diet: Chicken, Cracked Pearled Barley, Brown Rice, Brewers Rice, Whole Grain Wheat

These look like foods from different planets. z/d uses corn starch and hydrolyzed proteins to eliminate allergens; Science Diet uses standard chicken and grains. z/d’s ingredient list is unusual but purposeful. Science Diet’s is conventional but packed with plant protein fillers further down.

Where Hill’s Prescription Diet z/d pulls ahead

z/d avoids corn gluten meal, soybean meal, and standard wheat — all common allergens and all present in Science Diet. The hydrolyzed proteins are a genuine scientific innovation for managing food allergies. Flaxseed, coconut oil, and fish oil create a strong fatty acid profile for skin health. Pressed cranberries add whole-food antioxidants. Dried beet pulp provides prebiotic fiber. The formula is cleaner in the supporting ingredient tier despite its unusual lead ingredients. Shop on Amazon →

Where Hill’s Science Diet holds its own

Whole chicken at position one provides intact, high-quality protein — something z/d can’t include by design. Barley and brown rice are nutritionally superior to corn starch. Science Diet includes FOS prebiotics. It costs substantially less and doesn’t require a veterinary prescription. For dogs without food allergies, Science Diet provides a more conventional, balanced nutrition profile with intact protein sources. Shop on Amazon →

The bottom line

Hill’s Prescription Diet z/d (B/76) scores higher than Science Diet (C/61), but this isn’t really a head-to-head choice. z/d is for dogs with confirmed food allergies who need hydrolyzed proteins. Science Diet is general maintenance. If your dog has allergies, z/d is one option; if not, consider Blue Buffalo (B/78) or Wellness Complete Health (B/82) for a non-prescription upgrade over Science Diet.